440 ECHINODERMATA ASTEROIDEA chap. 



Soon the bivalve is forced to relax its muscles and allow the 

 valves to gape. The edge of the stomach is then inserted between 

 the valves and applied directly to the soft parts of the prey 

 which is thus completely digested. When the Starfish moves 

 away nothing but the cleaned shell is left behind. If the bivalve 

 is small it may be completely taken into the stomach, and the 

 empty shell later rejected through the mouth, 



It was for a long time a puzzle in what way the bivalve was 

 forced to open. Schiemenz ^ has, however, shown that when the 

 Starfish folds itself in umbrella-like form over the prey it holds 

 on to the substratum by means of the tube-feet of the distal 

 portions of the arms, whilst, by means of the tube-feet belonging 

 to the central portions, it drags apart the valves by main force. 

 He has shown experimentally : (1) that whilst a bivalve may be 

 able to resist a sudden pull of 4000 grammes it will yield to a 

 pull of 900 grammes long continued; (2) that a Starfish can 

 exert a pull of 1350 grammes; (3) that a Starfish is unable to 

 open a bivalve unless it be allowed to raise itself into a hump, 

 so that the pull of the central tube-feet is at right angles to the 

 prey. A Starfish confined between two glass plates walked about 

 all day carrying with it a bivalve which it was unable to open. 



The lining of the stomach is found to consist very largely of 

 mucus-forming cells, which are swollen with large drops of mucus 

 or some similar substance. It used to be supposed that this 

 substance had some poisonous action on the prey and paralysed 

 it, but the researches of Schiemenz show that this is incorrect. 

 If when an Asterias is devouring a bivalve another be offered to 

 it, it will open it, but will not digest it, and the victim shows no 

 sign of injury but soon recovers. The cells forming the walls 

 of the pyloric sac and its appendages are tall narrow cylindrical 

 cells crowded with granules which appear to be of the nature of 

 digestive ferment. This substance flows into the stomach and 

 digests the captured prey. 



A very small amount of matter passes into the rectum and 

 escapes by the anus, as the digestive powers of the Starfish are 

 very complete. The rectal caeca are lined by cells which secrete 

 from the coelomic fluid a brown material, in all probability an 

 excretion, which is got rid of by the anus. 



' Mitth. des deiUscAen Seefischervereins, xii. 1896, p. 102, and J. Mar. Biol. Ass. 

 iv. 1895-97, p. 266. 



